A. Field of the Invention
The present invention relates to a method and apparatus for efficient recovery of hydrocarbons from subsea wells. More specifically, the apparatus operates to reduce the formation of hydrates, reduce backpressure on the well and prevent paraffin formation in the hydrocarbon recovery process.
B. Description of the Related Art
Recovery of oil and gas requires removal of the hydrocarbons from their subterranean location and transport to processing plants or storage containers. Offshore hydrocarbon production is particularly complicated by the traverse of the hydrocarbons from the sea bed to the sea surface because the additional backpressure on the well reduces the flow of hydrocarbons to the surface and because the hydrocarbons tend to cool as a result of heat loss to the surrounding sea water. When the unrefined hydrocarbons cool, solids tend to form on the inner walls of recovery pipelines, obstructing hydrocarbon flow and decreasing recovery efficiency. If cooling is extreme, as when the flow is temporarily stopped, the flowlines may become completely plugged.
Solids that form in hydrocarbon recovery pipelines are of two types, paraffins and hydrates. Hydrates are solids that form in gaseous hydrocarbon and water mixtures when the temperature falls below the hydrate formation temperature. Hydrate formation temperatures vary with pressure and the properties of the gas. Typical hydrate formation temperatures range from about 60.degree. F. to about 80.degree. F. at pressures normally present in hydrocarbon production pipelines. Paraffins are solids that form when oil cools below its "cloud point". In some cases the cloud point temperature can be over 100.degree. F. Since typical deep sea water temperatures range from about 30.degree. F. to 50.degree. F., heat exchange of hydrocarbons with cooler sea water can result in undesirable solid formation, particularly at low flow rates.
A variety of techniques can be used to prevent solid formation. For example, insulation of hydrocarbon recovery pipelines prevents heat exchange with sea water. However, it is well known that natural flow rates vary between different wells and for individual wells over time. Insulation alone is insufficient to prevent problems with paraffin formation when hydrocarbon flow through a recovery line is very slow or when there is no flow. Furthermore, circumstances may demand that the well be temporarily shut down, for example when a hurricane passes or when mechanical problems occur. If the flow rate slows enough to allow the hydrocarbon to cool below its hydrate formation temperature or cloud point, then solids will accumulate in the pipelines and hydrocarbon recovery efficiency will be reduced. It would be prohibitively expensive to insulate pipelines adequately enough to prevent solid formation under all circumstances. Furthermore, insulation provides no benefit in restarting hydrocarbon flow through obstructed pipelines.
Solid hydrate formation can also be inhibited by chemical additives such as methanol, ethylene glycol and the like, which are introduced directly into the hydrocarbon recovery pipelines. However, the cost of these inhibitors may make their continuous use uneconomical.
Another strategy for preventing solid formation involves adding heat to the recovered hydrocarbon. An example, of one such strategy is disclosed in U.S. Pat. No. 4,679,598, which describes a method of transporting subsea gas through a tube bundle. The bundle provides an insulating external casing, an internal heating tube for carrying a heated medium and at least one hydrocarbon production pipeline. Heating fluid provided by an external source is circulated through the bundle and is transmitted into the production flowline. However, this invention does not reduce backpressure on the well.
When solids form in a pipeline, one technique for clearing the line is "hot oiling the well". This is a method of melting out the solids by introducing heated oil from the surface into a partially occluded pipeline. The hot oil melts away the blockages. A system of this type is described in U.S. Pat. No. 4,911,240. This patent discloses an oil heater and storage tank and a pipeline leading from the storage tank to the wellhead. Hot oil is introduced into the wellhead, where it passes into the well string and dissolves paraffins and other deposits. The "hot oiling" process interrupts production and does not prevent the build up of paraffins in an ongoing manner. The oil recovery industry has not yet found a way to "hot oil" a well without interrupting hydrocarbon recovery. A process such as this would be more useful if it could be carded out during production and in a way that reduces backpressure on the well.
None of the previously described techniques can be used to reduce backpressure on a well. However, a variety of techniques are known to reduce back pressure on a well. For example, downhole pumps can be used to pump oil to the surface. The use of downhole pumps is described in U.S. Pat. No. 4,967,843, which discloses the use of a pump employed near the bottom of the well to force composite hydrocarbons (hydrocarbons containing gas and liquid) to the surface. The pump, which is powered by a pressurized fluid line drives hydrocarbons up through the inner one of a pair of concentric tubes. The annular space between the tubes is used to send the drive fluid to the pump. The utility of this invention is limited by the cooling of the hydraulic fluid which is recirculated from the platform down to the pump. Mixing the cooled fluid with crude oil, as taught, could stimulate the production of both hydrates and paraffin in the composite hydrocarbon flowline. In addition, the recirculated hydraulic fluid surrounds the inner flowline and operates as a heat sink, drawing heat from the newly recovered oil.
Another, more sophisticated recovery technique is disclosed in U.S. Pat. No. 4,705,114, which describes directing a composite hydrocarbon stream along the sea floor to directly beneath a platform where the stream is separated into gas and liquid streams. The gas and liquid streams are introduced into the annulus of a riser comprising dual concentric pipes. The gas flows upward through the annulus of the riser to the platform, where it is collected. The liquid hydrocarbon pool flows down the annulus by gravity and enters the sump of a well beneath the riser tube. A downhole pump located in the sump then forces the liquid hydrocarbon to the surface through the inner pipe of the riser. The invention provides for recirculation of recovered hydrocarbon which may be required to allow the pump to maintain in continuous operation when the recovered oil is depleted from the sump. This system offers an advantage in that separating gas from liquid eliminates problems with hydrate formation in the liquid dominated lines, while paraffin formation is not a problem in the gas dominated line. However, by removing the gas from the crude oil, it is more likely that paraffins will form in the liquid. Moreover, introducing the liquid into the lower section of the riser pipe where the surrounding sea water is the coolest and requiring it to drain into the sump of the inner pipe of the riser may allow the hydrocarbon liquid to cool to below the cloud point.
Another method for reducing backpressure on the well is taught in U.S. Pat. No. 5,199,496. Seawater flows into an aspirator that is connected to a oil production manifold. The oil is driven to a riser which utilizes a gas lift to urge the oil/sea water mixture to the surface. While this technology may be useful in reducing backpressure on the well, mixing of cold sea water with a heavy oil could stimulate paraffin and hydrate formation and consequent pipe blockages. Furthermore, sea water is not normally considered compatible with water produced from hydrocarbon pools because these mixtures have a propensity to form scale inside the hydrocarbon recovery lines. Leakage of the check valve by the seawater intake port, which could occur after substantial scale accumulation, could allow hydrocarbons to leak directly into the sea.
U.S. Pat. No. 4,154,297 describes a gas lift apparatus that includes a heat exchanger for exchanging heat from well fluids to the gas injected into a well. The invention prevents problems of local cooling caused by the rapid depressurization of the gas near the gas jet. This cooling could lead to solid formation around the gas jet and clogging of the pipeline. However, the invention operates by removing heat from the recovered hydrocarbon stream, thereby causing the same net loss of heat from these liquids.
The inventions considered previously provide advantages either in reducing the backpressure on the well or in maintaining the elevated temperature of the recovered hydrocarbons. However, a system that operates to reduce backpressure on the well while at the same time eliminating problems with solid formation would be particularly useful. Furthermore, the system should not be difficult to restart after extended shutdowns.